Skin problems are pervasive in society. People suffer from conditions ranging from the cosmetic, such as benign discoloration, to fatal ailments, such as malignant melanomas. Treatments range from cosmetic "cover-up" makeup to surgical excision.
In particular, the removal (or "skin peel") of an outer layer of skin is used to treat conditions such as acne, age spots (superficial regions of excess melanin), shallow lesions (e.g. actinic keratoses), and aged skin. FIG. 1 depicts a cross section of normal human skin. The depth of the outer layer, or epidermis, varies, with ranges typically from 50-150 .mu.m in thickness. The epidermis is separated from the underlying corium (dermis) by a germinative layer of columnar basal cells. The epidermal/dermal interface is characterized by undulations. The basal cells produce a continuing supply of keratinocytes, which are the microscopic components of the epidermis. Specialized cells called melanocytes, also reside in the basal cell layer and produce the pigment melanin. Although some of the melanin migrates toward the surface of the skin with the keratinocytes, the greatest concentration of melanin remains in the basal cell layer. The uppermost layer of the dermis, which is adjacent to the basal cell layer, is known as the papillary dermis, and the papillae range in width from 25-100 .mu.m, separated by rete ridges ("valleys") of comparable width.
Removal of the epidermis eliminates superficial sun damage, including keratoses, lentigenes, and fine wrinkling. Removal of the most superficial portions of the dermis, i.e. the uppermost papillary dermis, eliminates solar elastosis and ameliorates wrinkling, with little or no scarring.
One treatment that is currently very popular uses a short pulse carbon dioxide (CO.sub.2) laser to coagulate a layer of skin to a depth of .about.50-100 .mu.m /pulse. This treatment is sometimes referred to as a "laser peel." CO.sub.2 laser radiation (in the 9-11 .mu.m region of the infrared) is strongly absorbed by water (contained in all tissue). When the energy/unit volume absorbed by the tissue is sufficient to vaporize the water, a microscopically thin layer of tissue at the surface of the irradiated region is rendered necrotic; (see e.g., R. M. Adrian, "Concepts in Ultrapulse Laser Resurfacing," URL--http://www.lasersurgery.com/physicians.html (1996)). The skin is coagulated on the surface of the region irradiated by the infrared beam from the CO.sub.2 laser. After irradiation, the dehydrated, necrotic surface layer is mechanically removed, and additional irradiation takes place, this process being repeated until the desired depth of tissue is removed. In medical terms, tissue is removed with less collateral damage than with other modalities, e.g., liquid nitrogen, cautery, chemical peels; (see e.g., E. V. Ross et al, "Long-term results after CO.sub.2 laser skin resurfacing: a comparison of scanned and pulsed systems," J Amer Acad Dermatology 37: 709-718 (1997)).
The radiation features that result in reduced collateral damage are: a wavelength that is strongly (optically) absorbed; and a pulse duration that is short compared to the time for deposited energy to diffuse into the surrounding tissue; (see e.g., R. J. Lane, J. J. Wynne, and R. G. Geronemus, "Ultraviolet Laser Ablation of Skin: Healing Studies and a Thermal Model," Lasers in Surgery and Medicine 6: 504-513 (1987)). The short pulse CO.sub.2 laser, with an absorption coefficient in tissue of .about.1/50 .mu.m.sup.-1 and a pulse duration of .about.10-100 ns, leads to a reduction of collateral damage as compared to a continuous wave (cw) CO.sub.2 laser or lasers at other visible and near infrared wavelengths. But clinical results on patients treated for the elimination of wrinkles with the short pulse CO.sub.2 laser show an abundance of undesirable aftereffects, including erythema (red, inflamed skin) and crusting, with the possible formation of scar tissue and dyspigmentation. (see e.g., Andrew Bowser, "Addressing Complications from Laser Resurfacing-Deep scarring and insufficient follow-up are among causes cited," Dermatology Times 18, No. 9: 50-51 (September 1997)). One problem with the pulsed CO.sub.2 laser is that the radiation absorption depth is not shallow enough. Another is the deposited laser energy is not sufficiently removed from the surface to completely prevent collateral damage. As a result, this modality for "superficial skin peel" can be relatively painful, often requiring general anesthesia and a prolonged recovery period.
A newer treatment that is gaining in popularity uses a pulsed erbium YAG (Er:YAG) laser, emitting radiation at 2.94 .mu.m in the infrared, where water absorption is even stronger than at CO.sub.2 wavelengths. Er:YAG light is approximately 10 times more strongly absorbed in skin than CO.sub.2 laser light. When compared to the effect of CO.sub.2 laser irradiation, a shallower layer of skin absorbs the radiation and is vaporized and ablated from the surface, leaving a thinner thermally damaged and coagulated layer adjacent to the removed tissue. Damage has not been observed to exceed a depth of .about.50 .mu.m of collagen denaturation. (See e.g., R. Kaufmann and R. Hibst, "Pulsed Erbium:YAG Laser Ablation in Cutaneous Surgery," Lasers in Surgery and Medicine 19: 324-330(1996)).
Treatment with the Er:YAG laser rejuvenates skin, with less pain, less inflammation, and more rapid healing than treatment with the CO.sub.2 laser. The depth of penetration with the Er:YAG laser, being shallower, does not thermally stimulate new collagen growth as much as the CO.sub.2 laser, so fine wrinkles are not eradicated as effectively. Dermatologists and cosmetic surgeons are finding the Er:YAG laser preferable for younger patients who have superficial skin damage but less wrinkling, while the CO.sub.2 laser is thought to be preferable for older patients who want to have fine wrinkles removed around the lips and the eyes (see e.g., Betsy Bates, "Dermatologists Give Er:YAG Laser Mixed Reviews," Skin & Allergy News 28, No. 11: 42 (November 1997)). Although the depth of penetration is shallower and the skin is actually ablated rather than just rendered necrotic, the ablation depth and the depth of coagulated skin limits the precision with which the Er:YAG can remove epidermal tissue without damaging the underlying papillary dermis. And while pain is lessened, many patients still require some sort of anesthesia during treatment and the application of a topical antibiotic/antimicrobial agent following treatment to prevent infection during healing.
It is also generally known in the art to use ultraviolet wavelength lasers for medical and dental applications. See e.g., U.S. Pat. No. 4,784,135, issued Nov. 15, 1988, entitled "Far Ultraviolet Surgical and Dental Procedures," by Blum, Srinivasan, and Wynne. See also U.S. Pat. No. 5,435,724, entitled "Dental Procedures and Apparatus Using Pulsed Ultraviolet radiation," issued Jul. 25, 1995 to Goodman, Wynne, Kaufman and Jacobs.
The need remains for an apparatus and method which provides for skin resurfacing in a painless environment with exquisite control and markedly decreased morbidity by eliminating erythema and scarring. There is also a need for an apparatus and method of using ultraviolet (uv) light, delivered in a finely-controlled, countable number of short pulses with sufficient fluence to ablate skin, to remove thin layers of skin at the site of irradiation, with exquisite lateral precision and depth control, resulting in minimal damage to the skin surrounding and underlying the ablated area. The present invention addresses these needs.
There is also a need for an apparatus and process which enables a controlled removal of the epidermis down to the papillary dermis, adapting to the undulations of the papillary dermis. The present invention addresses such a need.